Apparatus for covering an object with a layer of powder

ABSTRACT

This invention concerns a spray nozzle fed with the powder conveyed in an air stream constructed so that expansion of the air conveying the powder is effected in the nozzle and the spread of the spray jet issuing from the nozzle is adjusted by regulating the flow of an auxiliary jet of air which whirls about the air stream conveying the powder. In a specific embodiment the nozzle comprises a conduit having a downstream portion of larger cross-section than the cross-section of an upstream portion thereby forming an expansion chamber in the nozzle; and the jet of whirling air is admitted to the nozzle conduit downstream of the expansion chamber. A conductive core is mounted coaxially in the nozzle conduit and is adapted to be connected to a source of high voltage. The ionization means for ionizing the air conveyed powder are at the outlet of the nozzle.

United States Patent Fabre [4 1 Apr. 25, 1972 [541 APPARATUS FORCOVERING AN 3,351,285 11/1967 1110101118 ..239/3 OBJECT WITH A LAYER OFPOWDER 3,473,735 10/1969 Drum 3,517,262 6/1970 Juvinall et a1. 1 1 Film,Grenoble France 3,326,182 6/1967 1110116 ..117/93.4 [73] Assignee:Tunzini-Sames, Grenoble, France FOREIGN PATENTS OR APPLICATIONS [2211969 722,608 11/1965 Canada ..117/17 2 AWL 333,793 1,026,413 4/1966Great Britain ..118/629 Primary Examiner-William D. Martin 8"Application Assistant Examiner-Raymond M. Speer Dec. 17, 1968 P1111166..5302 Bnsebms 52 us. 01 ..317/3, 117/17, 239/3, [57] ABSTRACT 239/298This invention concerns a spray nozzle fed with the powder [51] Int. Cl...B05b 5/02 conveyed in an air stream constructed so that expansion ofthe [58] Field of Search ..117/17,93.4,93.44, 104; air conveying thepowder is effected in the nozzle and the 118/621, 629; 239/3, 298;317/3, 4 spread of the spray jet issuing from the nozzle is adjusted byregulating the flow of an auxiliary jet of air which whirls about [56]References Ci the air stream conveying the powder. In a specificembodiment the nozzle comprises a conduit having a downstream UNITEDSTATES PATENTS portion of larger cross-section than the cross-section ofan upstream portion thereby forming an expansion chamber in the if; 2 gg 4 nozzle; and the jet of whirling air is admitted to the nozzle con-3/1939 aascfe 239/298 duit downstream of the expansion chamber. Aconductive Gusm et a core is mounted coaxially in the nozzle conduit andis adapted 23021289 1942 Bfamsmn'cookm "117/17 to be connected to asource of high voltage. The ionization 21569125 1 9/1951 Nleburg,239/298 means for ionizing the air conveyed powder are at the outlet of2,894,691 7/1959 Sed1acs1k.. 117/93.44 the nozzle 3,248,606 4/1966Fraser ..317/3 3,296,015 1/1967 Juvinall et al ..1 17/93.44 5 Claims, 1Drawing Figure "I I \ZZ4 4t\\\\ 9 712 Q'QLIIIIIIIII The presentinvention relates to a method of and apparatus for covering an objectwith a layer of powder wherein a cloud of powder is directed towards thesaid object to be coated across a zone of high ionization created by anelectric member carried to a high potential with respect of that of theobject to be coated. in a particular embodiment of this method, asecondary jet of air of a whirling nature is caused about the axial jetof air conveying the powder, which allows this latter to spreadconsiderably in a direction transverse to the direction towards theobject, therefore allowing a more uniform and rapid covering of anobject to be obtained in numerous applications.

However this type of method is limited in use for certain objects havingparticular shapes and more particularly, in the case of objects havinghollow shapes in the center of convex shapes it presents difficulties incoating these two very different shapes. In fact, if one is content touse a spray gun having a direct axial jet, the cloud of powder may bemoved so as to be deposited in the hollows, but on the contrary a verylimited coating is obtained on account of the surface. In such cases oneis usually obliged to make do with touching-up with the aid of differentspray guns, by using firstly a spray gun with a large opening and then adirective spray gun having a small opening, or vice versa.

The present invention has for an object to provide a method andapparatus which allows this drawback to be reduced or avoided.

The method according to this invention effects, in the spray nozzle, anexpansion of the current of air conveying the owder and the spread ofthe sprayed jet is adjusted to be larger or smaller by adjusting theflow of an auxiliary jet of air coaxially whirling about the air streamconveying the powder between a flow resulting in the formation of adirect jet and a higher flow resulting in a wide spread of the spray.Using these methods allows one direct jet to be provided at a relativelylow axial speed on account of the expansion which has taken place, whilethe sprayed jet may easily be adapted at any time, and while operating,to the surface form during a coating.

The invention also provides apparatus for carrying out the method,comprising a nozzle traversed by a conduit for a gaseous streamconveying the powder, ionization means at the outlet from the saidnozzle, means for forming a jet of air coaxially whirling about the airstream conveying the powder and wherein the nozzle conduit has adownstream portion of a sub stantially larger cross section than that ofan upstream portion so as to form an expansion chamber.

The invention will now be further described, by way of example, withreference to the accompanying drawing, which is a schematic sectionalview of one embodiment of atomization nozzle according to the invention.

Referring to the drawing, an atomization nozzle comprises a feed tube 1for a current of air conveying powder from a powder source (not shown)to the nozzle. The nozzle itself has an upstream nozzle conduit part 2made of a conductive material, followed by a downstream nozzle conduitpart 3 made of an insulating material, and an inner core 4 arranged soas to define an annular channel between these two members 3 and 4. Thecore 4 is conductive and widens into a radial edge 5 forming anelectrode and is terminated at its downstream end by an insulating domedhead member 6, the maximum diameter of which is less than that of thediameter of the edge 5. The core 4 is supported from the part 2 by arms7 The nozzle conduit part 3 terminates at a rounded outer edge 8 whichflares outwardly and incorporates perforations or passages 9 discharginginto a zone situated at the beginning of the edge 8. These passages 9communicate with an outer annular conduit 10 formed between the outerface of the nozzle part 3 and an elongated threaded nut 11 securing thenozzle on the spray gun body 14. in this embodiment the passages 9 areinclined in transverse planes with respect to the radial directions soas to abut in a substantially tangential way the inner surface of thenozzle conduit part 3. They are also, as can be seen from the drawing,slightly inclined forwardly in the axial direction, for example at 15.

The body of the spray gun 14 is an insulating hollow barrel throughwhich passes the powder feed tube 1 and the high voltage cable 15, whilethe interstitial space in the barrel 14 is used to conduct the air whichis fed to the passages 9.

The nozzle conduit has a cross section increasing substantially betweenthe inlet end 15 defined by the cylindrical tip 2a of the conduit part 2and the annular conduit 13 situated between the conduit part 3 and thecore 4. This increase in cross section occurs in a progressive manner byproviding the conduit part 2 with a conically diverging portion 2b.Advantageously the core 4 has an upstream pointed part 4a which assiststhe flow of air conveying the powder and reduces any obstruction of thislatter. 7

In operation, the air conveying the powder is fed in through the conduit1 and this air expands largely in the annular chamber 13 between thecore 4 and the conduit part 3. This expansion causes a reduction inspeed in the axial direction which favors a particularly suitableelectrostatic directive spray, that is to say'without too much axialspeed. The current of air whirling about the axial stream of air inducesa spreading of the air jet conveying the powder which allows thecovering of large areas. This spreading of the air stream conveying thepowder may be adjusted while operating, according to the form and designof the object to be coated. This can be achieved simply by varying theflow of air introduced into the annular conduit 10, for example by meansof a valve. It results that the shape of the powder spray may beadjusted from a practically cylindrical form, such as is shown at 12 atthe outlet of the nozzle conduit 3, to a very pronounced spread whichcorresponds substantially to the cone formed by the tangent to the area8 a little upstream of its free end. By reason of this arrangementoptimum condition can be obtained in the coating of bodies of veryvarying shapes and more particularly of large shape having hollowsurfaces. It is sufficient for the operator to act on the air adjustingvalve in the conduit 10 to ensure a larger or smaller spreading of theair jet.

it will be understood that the invention is not limited to theembodiment described and shown. Thus the nozzle may be produced entirelyor partly of a conductive material.

lclaim:

1. Apparatus for coating an object with a layer of powder, saidapparatus comprising a nozzle defining a conduit for a gaseous streamconveying the powder, the downstream portion of said nozzle conduitbeing substantially larger cross section than its upstream portion, aconductive core mounted coaxially in said nozzle conduit and adapted tobe connected to a source of high voltage, ionization means at the outletof said nozzle, and means for forming an adjustable spray of aircoaxially whirling about the gaseous stream conveying the powder, inwhich said spray-forming means discharge into said downstream portion ofsaid nozzle conduit.

2. Apparatus according to claim 1, wherein the nozzle has an outletportion defining a terminal part of the nozzle conduit which outletportion widens outwardly to a rounded edge, and the means for forming aspray of coaxially whirling air comprises a plurality of passages insaid noule discharging into said conduit upstream of said terminal part.

3. Apparatus according to claim 2, wherein the passages are all inclinedto intersect the conduit substantially tangentially.

4. Apparatus according to claim 3, wherein the passages are slightlyinclined forwardly.

5. Apparatus as claimed in claim 1 in which the portion of said nozzledefining the downstream portion of said conduit is made of an electricalinsulating material, and the portion of said nozzle defining theupstream portion of said conduit is made of an electrically conductivematerial.

1. Apparatus for coating an object with a layer of powder, saidapparatus comprising a nozzle defining a conduit for a gaseous streamconveying the powder, the downstream portion of said nozzle conduitbeing substantially larger cross section than its upstream portion, aconductive core mounted coaxially in said nozzle conduit and adapted tobe connected to a source of high voltage, ionization means at the outletof said nozzle, and means for forming an adjustable spray of aircoaxially whirling about the gaseous stream conveying the powder, inwhich said sprayforming means discharge into said downstream portion ofsaid nozzle conduit.
 2. Apparatus according to claim 1, wherein thenozzle has an outlet portion defining a terminal part of the nozzleconduit which outlet portion widens outwardly to a rounded edge, and themeans for forming a spray of coaxially whirling air comprises aplurality of passages in said nozzle discharging into said conduitupstream of said terminal part.
 3. Apparatus according to claim 2,wherein the passages are all inclined to intersect the conduitsubstantially tangentially.
 4. Apparatus according to claim 3, whereinthe passages are slightly inclined forwardly.
 5. Apparatus as claimed inclaim 1 in which the portion of said nozzle defining the downstreamportion of said conduit is made of an electrical insulating material,and the portion of said nozzle defining the upstream portion of saidconduit is made of an electrically conductive material.